I can't really speak to the way the liner works on your boat. On mine the liner and the hull look pretty independent in that area, so I assume that there is a reason for that. It might not make any difference at all; maybe I'm just being a bit paranoid. However, I do know that stiffer is not always better; fiberglass boats often flex a bit for a reason. On a 1964 model, that may not have been a consideration, but you would have to ask a real boat designer/architect to be a bit more sure.

I think one thing you really want to avoid is any sort of a "point load" on the hull. In other words, I wouldn't just transfer the load from the compression post to the hull without spreading it out over a pretty wide area. Having a little bit of the liner deform is one thing; having the hull deform would be far more problematic.

Simply stiffening the beam may be the way to go. That way the design essentially remains the same. How to go about that? Ay, there's the rub!